Are there any decentralized examples of AI systems with blockchain technology?
Swarm intelligence is the term for systems where relatively simple agents work together to solve a complicated problem in a decentralized fashion.

In general, distributed computing methods are very important for dealing with problems at scale, and many of them embrace decentralization in a deep way. (Given the reality of hardware failure and the massive size of modern datasets relative to individual nodes, the less work is passed through a central bottleneck, the better.) 

While there are people interested in doing computation on the blockchain, it seems to me like it's unlikely to be competitive with computation in dedicated clusters (like AWS).